Process for casting single crystal shapes



NOV. 24, 1970 w, s ETAL PROCESS FOR CASTING SINGLE CRYSTAL SHAPES Filed March ;20, 1968 United States Patent O 3,543,284 PROCESS FOR CASTING SINGLE CRYSTAL SHAPES Larry W. Sink, Rocky Hill, and Bernard H. Kear, Madlson, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Mar. 20, 1968, Ser. No. 714,743 Int. Cl. B22d 27/04 US. Cl. 164-60 6 Claims ABSTRACT OF THE DISCLOSURE A process for use in casting materials of single crystal in single and complex shapes wherein the process produces an increased heat extraction rate from the cast part, the rate also being compatible with a rate wh ch promotes the fastest growth consistent with forming sound single crystal materials.

BACKGROUND OF THE INVENTION This invention relates to a process for the casting of materials of simple or complex single crystal shapes.

In casting alloy single crystals, it is often desirable that parts or specimens having very thin sections, large length-to-thickness ratios, or large variations in thickness along the length of the part be cast. This has been difficult to do because the methods and geometry of the molds used for casting single crystal shapes have limited the control of heat extraction during the growth of the single crystal. The conventional methods used heretofore have required the extraction of heat from the solidifying crystal through a double-turn or helical constriction at the bottom of the mold. As a result, heat has not had a ready path to the chill plate, and consequently the heat extraction rate has been lower than required for the close control of growth rate in thin sections or sections of varying thickness. Therefore, to produce relatively long or thin sections; or, complicated shapes containing thin sections; it is necessary to provide a method which will permit a more efficient heat extraction rate than is obtainable with previous methods or mold designs. In addition to providing better control of heat extraction rate, the method used should simultaneously promote the fastest growth rate consistent with single crystal growth.

The present invention herein described provides a process for casting alloy single crystal shapes while satisfying the foregoing criteria.

SUMMARY OF THE INVENTION It is a primary object of this invention to provide a novel process for the casting of material of simple or complex single crystal shapes, the process being such that the rate of solidification is increased in the casting mold by virtue of a more efiicient extraction of heat therefrom. Additionally, this process provides a rate which is compatible with a rate which promotes the fastest growth consistent with forming sound single crystals.

The process of the present invention is employed in a preferred mold construction wherein at least one smaller mold is positioned within a main mold. The main mold comprises a ceramic tube, one open end of which is placed on a chill plate. The geometry of this smaller mold is such that it contains a small opening therein, this opening generally being positioned at the bottom of the smaller mold and providing a communication means between the smaller mold and the main mold. It is pointed out that any shape mold can be used; that is a simple shape such as bars, rods, sheets, or wires; or a complex shape such as blades, vanes, or hollow components. In a preferred use of this process, the smaller molds are positioned within the main mold such that the small opening in the smaller mold is above the chill plate, a preferred distance being one-half inch.

After molten metal is poured into the main mold, the body of the main mold having been preheated above the melting point of the alloy, the directional solidification process begins, and the liquid-solid interface moves away from the chill plate with a particular columnar oriented structure. Hence, when the liquid-solid interface reaches the small opening of the smaller mold, only one grain can grow upward through the length of the said opening, introducing single crystal growth into the small mold. The axis of orientation of material within the smaller mold is a function of the axis of orientation of the directionally solidified metal, the orientation of the opening in the smaller mold and/or the orientation of the smaller mold within the main mold.

By employing the process herein described, it becomes clear that the only restriction to heat transfer from the molten metal exists in the material forming the smaller mold. More specifically, the material forming the small opening in the small mold is a restriction to heat transfer from the metal in the mold to the chill plate. The process described to this point provides an improvement in heat extraction over previously used methods of casting single crystals because of much simplified heat flow path. However, the process provides a further increase in the rate of heat extraction from the smaller mold. The upward advance of the liquid-solid interface within the smaller mold will lag behind that of the larger mold since there is no restriction to vertical heat extraction in the larger mold. Moreover, since in the present process the metal in the larger mold is in direct contact with the outside of the smaller mold, heat flows through the walls of the smaller mold and then downward to the chill through the unrestricted volume of the larger mold. The rate of growth of the solid metal in the larger mold can then be adjusted by regulating the heat input through the heater at the top of the mold to control the heat extraction from the smaller mold to provide the fastest growth rate consistent with the growth of sound single crystal material, largely independent of the shape or cross-section of the smaller mold. Significantly, the volume or mass within the main mold is greater than the volume or mass Within the smaller molds. Since the liquid-solid interface within the main mold initially moves at a faster rate than the liquid-solid interface within the smaller mold, the latter is at a higher temperature at any point along its length. Therefore, heat from the smaller mold is transferred to the larger mass within the main mold, establishing a relationship between the growth rate of the single crystal within the smaller mold and the growth rate of the directionally solidified alloy in the main mold. Since this mass is substantial with respect to the mass within the smaller mold, a more efiicient rate of heat extraction is possible, this rate being largely independent of the shape of the specimen being cast in the smaller mold. In summary, it is reiterated that the process described herein provides a method of efficiently extracting heat from a mold for casting materials in simple or complex single crystal shapes. This is accomplished by providing a minimum restriction to heat transfer from the casting mold to a chill plate, by providing additional heat paths for the transfer of heat from the casting mold, and by providing a growth rate relationship between the growing single crystal and a much larger mass of a growing directionally solidified ingot.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. -1 is a schematic top sectional view of the preferred FIG. 2 is a vertical section of a preferred mold construction for use with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the present preferred embodiments of the present invention, a novel form of mold geometry is shown in FIGS. 1 and 2. The mold construction described herein is particularly suited for use with any of the so-called super alloys as described for example in the VerSnyder patent, US. No. 3,260,505 and having the same assignee as this application. As therein noted, these alloys are generally adapted for the process known as directional solidification. The mold construction herein described, in adddition to the disclosure contained in the VerSnyder patent, employs the technique of forming Monocrystaloys as described in Piearcey Pat. 3,494,709.

The mold construction described herein is a preferred mold construction and is the subject of copending application entitled A Mold Construction Forming Single Crystal Pieces, Ser. No. 714,589 filed Mar. 20, 1968, and assigned to the same assignee. As herein illustrated, one end of tubular mold 4, compatible for use with the procedure described in the VerSnyder patent, is placed on a relatively cool, heat conductive and preferably water cooled plate 6. Tubular mold is preferably made from a ceramic material from a conventional slurry of alumina or other high melting point refractory material in accordance with standard shell-molding techniques; and water for the chill plate 6 is carried through conduits 8. As illustrated, one end of tube 4 rests on chill plate 6 and cooperates to form an enclosed cavity 10. The end of tube 4 opposite chill plate 6 is open to receive molten metal.

Surrounding cavity 10 are the means for heating the mold to the desired temperature for casting. Preferably the cavity is surrounded by an electrical resistance heating coil 12 supplied with variable electrical current. Alternately, the cavity is surrounded by a graphite susceptor, not shown, and this in turn is surrounded by an induction coil supplied with high frequency electric current as is usual in an induction furnace. Prior to casting, the mold is heated to a desired temperature by supplying current to coil 12 and when the desired temperature has been attained, molten metal, heated to the proper temperature for casting, is poured into cavity 10. The chill plate 6 is maintained at a relatively cool temperature by means of water circulating through conduits 8 so as to etsablish a temperature gradient within the molten metal filling cavity 10 as the metal solidifies.

In the present embodiment, a plurality of individual molds 16, 18, and 22 are positioned around the inner periphery of surface 23 of tube 4. Each of these molds has a small opening 24, herein illustrated to be at the bottom of each mold, and is spaced above chill plate 6. Now as the metal begins to solidify it has a controlled columnar structure and by providing a small opening such as 24 in each of the individual molds 16, 18, 20 and 22, growth of a single crystal is promoted therein. Therefore, what is produced is in effect a large mass of controlled columnar structure surrounding relatively smaller masses of single crystal. A significant feature about the construction of the individual molds is that the small opening 24 in each of the molds is the only restriction to heat transfer from each of the molds to chill plate 6. As a result of this, this type mold geometry would permit the heat Within the mold to be extracted at a faster rate than the previous mold constructions which use a double turn restriction as illustrated in the Piearcey Pat. 3,494,709.

As hereinbefore noted, the volume of tube 4 is substantially greater than the volume within each of molds 16, 18, 20 and 22. Therefore, a relatively greater mass surrounds molds 16, 18, 20 and 22 than is interior thereof. Now as the directional solidification proceeds, the liquidto-solid interface moves away from chill plate 6. As a result of the restriction to heat transfer from the individual molds 16, 18, 20 and 22 to chill plate 6 caused by opening 24, the liquid-solid interface interior of molds 16, 18, 20 and 22 is slower moving than that of the mass surrounding the molds.

In other words, any distance from the chill plate, the solidification of the substantial mass within the tube will be higher than the solidification within molds 16, 18, 20 and 22. This also means that the mass surrounding the molds is at a lower temperature, and heat will be extracted laterally from molds 16, 18, 20 and 22. By removing heat from molds 16, 18, 20 and 22 at this increased or more efficient rate faster single crystal growth is promoted within the molds and smaller and/ or more complex shapes can be produced.

In each of FIGS. 1 and 2, the individual molds are shown positioned above the chill plate. This is of importance in that it has been found that during the directional solidification process, the solidified metal adjacent the chill plate has a random crystal orientation. Accordingly, to get a single crystal with a preferred orientation, the individual molds should be positioned above the chill plate a distance sufiicient to avoid this growth zone. It is clear that individual molds may be positioned at the same height above the chill plate or they may be placed in a stepped arrangement thereabove. However, it has been found that an optimum spacing from the chill plate to the lowest individual mold should be about one-half inch.

It is to be understood that the invention is not limited to the embodiments herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

We claim:

1. The process of casting materials in single crystal shapes comprising:

providing a main mold comprising a ceramic tube on a chill plate; positioning at least one smaller mold in the interior of the main mold, the smaller mold having restriction means in communication with the main mold; spacing the restriction means above the chill plate; filling the main mold and smaller mold With molten metal; cooling the molten metal and unidirectionally solidifying the molten metal in a direction away from the chill plate;

growing a single crystal material through the restriction means within the smaller mold; and

cooling the molten metal in the main mold at a different rate than that in the smaller mold and thereby providing different growth rates between the liquidsolid interface in the main mold and in the smaller mold, and hence providing a heat transfer relationship therebetween.

2. The process of casting materials as in claim 1 including:

positioning the means for communication between the small mold and the main mold at the bottom of the small mold.

3. The process of casting materials as in claim 1 including:

promoting rates of liquid-solid interface in the main mold and small mold such that the rate within the small mold is controlled by the rate in the main mold.

4. The process of casting materials in single crystal shapes comprising:

providing a main mold comprising a ceramic tube on a chillplate; positioning at least one smaller mold in the interior of the main mold, the smaller mold including restriction means in communication with the main mold; positioning the restriction means at the bottom of the smaller mold; positioning the bottom of the smaller mold above the chill plate;

spacing the restriction means above the chill plate;

filling the main mold and the smaller mold with molten metal;

cooling the molten metal and unidirectionally solidifying the molten metal in a direction away fromthe chill plate with a preferred axis;

positioning the smaller mold and communicating means within the main mold so that the axis of orientation of the anisotropic material found therein is parallel to the preferred axis;

growing a single crystal material in the smaller mold through the restriction means;

cooling the molten metal in the main mold at a different rate than that in the smaller mold; and

restricting the removal of heat from the metal in the 1' smaller mold so that the rate of growth therewithin is dependent on the rate of growth within the main mold, the rate of growth being controlled so that a heat transfer relationship exists therebetween.

5. The .process of casting materials as in claim 4 including:

positioning the bottom of the mold and hence the communicating means one-half inch above the chill plate. 6. The process of casting materials as in claim 4 including:

controlling the rates of growth of the liquid-solid interface in the main mold and small mold by cooling such that the rate of growth within the small mold is controlled by the rate of growth in the main mold.

References Cited UNITED STATES PATENTS 3,260,505 7/ 1966 Ver Snyder.

3,342,455 9/1967 Fleck et al. 253-77 3,376,915 4/1968 Chandley.

3,417,809 12/1968 Sink 164127 J. SPENCER OVERHOLSER, Primary Examiner R. S. ANNEAR, Assistant Examiner US. Cl. X.R. 

